13,022 research outputs found
Quadrupole-scissors modes and nonlinear mode coupling in trapped two-component Bose-Einstein condensates
We theoretically investigate quadrupolar collective excitations in
two-component Bose-Einstein condensates and their nonlinear dynamics associated
with harmonic generation and mode coupling. Under the Thomas-Fermi
approximation and the quadratic polynomial ansatz for density fluctuations, the
linear analysis of the superfluid hydrodynamic equations predicts excitation
frequencies of three normal modes constituted from monopole and quadrupole
oscillations, and those of three scissors modes. We obtain analytically the
resonance conditions for the second harmonic generation in terms of the trap
aspect ratio and the strength of intercomponent interaction. The numerical
simulation of the coupled Gross-Pitaevskii equations vindicates the validity of
the analytical results and reveals the dynamics of the second harmonic
generation and nonlinear mode coupling that lead to nonlinear oscillations of
the condensate with damping and recurrence reminiscent of the Fermi-Pasta-Ulam
problem.Comment: 10 pages, 5 figures, revtex
Dilute-Bose-Gas Approach to ground state phases of 3D quantum helimagnets under high magnetic field
We study high-field phase diagram and low-energy excitations of
three-dimensional quantum helimagnets. Slightly below the saturation field, the
emergence of magnetic order may be mathematically viewed as Bose-Einstein
condensation (BEC) of magnons. The method of dilute Bose gas enables an
unbiased quantitative analysis of quantum effects in three-dimensional
helimagnets and thereby three phases are found: cone, coplanar fan and an
attraction-dominant one. To investigate the last phase, we extend the usual BEC
approach so that we can handle 2-magnon bound states. In the case of 2-magnon
BEC, the transverse magnetization vanishes and long-range order occurs in the
quadrupolar channel (spin-nematic phase). As an application, we map out the
phase diagram of a 3D helimagnet which consists of frustrated J1-J2 chains
coupled by an interchain interaction J3.Comment: 4pages, 3figures, International Conference on Magnetism (ICM) 2009
(Karlsruhe, Germany, July 26-31, 2009)
Symmetry Breaking and Enhanced Condensate Fraction in a Matter-Wave Bright Soliton
An exact diagonalization study reveals that a matter-wave bright soliton and
the Goldstone mode are simultaneously created in a quasi-one-dimensional
attractive Bose-Einstein condensate by superpositions of quasi-degenerate
low-lying many-body states. Upon formation of the soliton the maximum
eigenvalue of the single-particle density matrix increases dramatically,
indicating that a fragmented condensate converts into a single condensate as a
consequence of the breaking of translation symmetry.Comment: 4 pages, 4 figures, revised versio
Topological defect formation in quenched ferromagnetic Bose-Einstein condensates
We study the dynamics of the quantum phase transition of a ferromagnetic
spin-1 Bose-Einstein condensate from the polar phase to the broken-axisymmetry
phase by changing magnetic field, and find the spontaneous formation of spinor
domain walls followed by the creation of polar-core spin vortices. We also find
that the spin textures depend very sensitively on the initial noise
distribution, and that an anisotropic and colored initial noise is needed to
reproduce the Berkeley experiment [Sadler et al., Nature 443, 312 (2006)]. The
dynamics of vortex nucleation and the number of created vortices depend also on
the manner in which the magnetic field is changed. We point out an analogy
between the formation of spin vortices from domain walls in a spinor BEC and
that of vortex-antivortex pairs from dark solitons in a scalar BEC.Comment: 10 pages, 11 figure
Spin textures in rotating two-component Bose-Einstein condensates
We investigate two kinds of coreless vortices with axisymmetric and
nonaxisymmetric configurations in rotating two-component Bose-Einstein
condensates. Starting from the Gross-Pitaevskii energy functional in a rotating
frame, we derive a nonlinear sigma model generalized to the two-component
condensates. In terms of a pseudospin representation, an axisymmetric vortex
and a nonaxisymmetric one correspond to spin textures referred to as a
"skyrmion" and a "meron-pair", respectively. A variational method is used to
investigate the dependence of the sizes of the stable spin textures on system
parameters, and the optimized variational function is found to reproduce well
the numerical solution. In the SU(2) symmetric case, the optimal skyrmion and
meron-pair are degenerate and transform to each other by a rotation of the
pseudospin. An external rf-field that couples coherently the hyperfine states
of two components breaks the degeneracy in favor of the meron-pair texture due
to an effective transverse pseudomagnetic field. The difference between the
intracomponent and intercomponent interactions yields a longitudinal
pseudomagnetic field and a ferromagnetic or an antiferromagnetic pseudospin
interaction, leading to a meron-pair texture with an anisotropic distribution
of vorticity.Comment: 14 pages, 15 figure
Finite-Size Scaling Analysis of the Eigenstate Thermalization Hypothesis in a One-Dimensional Interacting Bose gas
By calculating correlation functions for the Lieb-Liniger model based on the
algebraic Bethe ansatz method, we conduct a finite-size scaling analysis of the
eigenstate thermalization hypothesis (ETH) which is considered to be a possible
mechanism of thermalization in isolated quantum systems. We find that the ETH
in the weak sense holds in the thermodynamic limit even for an integrable
system although it does not hold in the strong sense. Based on the result of
the finite-size scaling analysis, we compare the contribution of the weak ETH
to thermalization with that of yet another thermalization mechanism, the
typicality, and show that the former gives only a logarithmic correction to the
latter.Comment: 5 pages, 3 figure
Oscillons: an encounter with dynamical chaos in 1953?
We present evidences that Ben F. Laposky (1914-2000) might have been the
first person who created a family of nonlinear analog circuits that allowed him
to observe chaotic attractors and other trademarks of nonlinear science as
early as 1953.Comment: accepted to Chao
Suzaku Confirms NGC~3660 is an Unabsorbed Seyfert 2
An enigmatic group of objects, unabsorbed Seyfert 2s may have intrinsically
weak broad line regions, obscuration in the line of sight to the BLR but not to
the X-ray corona, or so much obscuration that the X-ray continuum is completely
suppressed and the observed spectrum is actually scattered into the line of
sight from nearby material. NGC 3660 has been shown to have weak broad
optical/near infrared lines, no obscuration in the soft X-ray band, and no
indication of "changing look" behavior. The only previous hard X-ray detection
of this source by Beppo-SAX seemed to indicate that the source might harbor a
heavily obscured nucleus. However, our analysis of a long-look Suzaku
observation of this source shows that this is not the case, and that this
source has a typical power law X-ray continuum with normal reflection and no
obscuration. We conclude that NGC 3660 is confirmed to have no unidentified
obscuration and that the anomolously high Beppo-SAX measurement must be due to
source confusion or similar, being inconsistent with our Suzaku measurements as
well as non-detections from Swift-BAT and RXTE.Comment: Accepted to PAS
X-ray Dust Scattering at Small Angles: The Complete Halo around GX13+1
The exquisite angular resolution available with Chandra should allow
precision measurements of faint diffuse emission surrounding bright sources,
such as the X-ray scattering halos created by interstellar dust. However, the
ACIS CCDs suffer from pileup when observing bright sources, and this creates
difficulties when trying to extract the scattered halo near the source. The
initial study of the X-ray halo around GX13+1 using only the ACIS-I detector
done by Smith, Edgar & Shafer (2002) suffered from a lack of sensitivity within
50'' of the source, limiting what conclusions could be drawn.
To address this problem, observations of GX13+1 were obtained with the
Chandra HRC-I and simultaneously with the RXTE PCA. Combined with the existing
ACIS-I data, this allowed measurements of the X-ray halo between 2-1000''.
After considering a range of dust models, each assumed to be smoothly
distributed with or without a dense cloud along the line of sight, the results
show that there is no evidence in this data for a dense cloud near the source,
as suggested by Xiang et al. (2005). Finally, although no model leads to
formally acceptable results, the Weingartner & Draine (2001) and nearly all of
the composite grain models from Zubko, Dwek & Arendt (2004) give poor fits.Comment: 8 pages, 6 figures, accepted for publication in Ap
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